In optical communications systems, to improve the minimum reception sensitivity of an optical receiver, a method is used in which input signal light is amplified by, for example, providing an optical preamplifier in a stage prior to the optical receiver. In the optical preamplifier, generally, output level control (ALC: automatic level control) is used to increase the input dynamic range of the optical receiver system (for example, the whole optical receiver apparatus including the optical preamplifier and optical receiver), whereby an optical signal having a given optical power is output to the subsequent optical receiver.
Hitherto, an erbium doped fiber amplifier (EDFA) has been generally used as the optical amplifier used in such an optical receiver system. However, since an EDFA has restrictions on the folding radius, reduction in size is difficult.
Hence, semiconductor optical amplifiers (SOAs) have been recently the focus of considerable attention. SOAs may be manufactured using equipment and a process similar to those for semiconductor laser devices, and have been developed as amplifiers which realize reduction in size, lower power consumption, and lower cost. SOAs are also expected to allow reduction in the size of a system by use of monolithic integration with other semiconductor devices or hybrid integration with other optical parts.
The ALC in an SOA is realized by monitoring the output light power of the SOA and performing feedback control of the drive current of the SOA. More specifically, in the ALC of an SOA, the signal light power included in the output light of the SOA is controlled to be at a given level by performing feedback control of the drive current so as to make the output light power of the SOA be at a given level. Thereby, signal light having a given level of an optical power is output to the subsequent optical receiver.
Note that in a feedback configuration using detection of an average, a signal light intensity controller has been conventionally proposed which may reduce a decrease in output signal intensity by detecting an output signal including amplified spontaneous emission (ASE), and thereby controlling the injected current. See, for example, Japanese Laid-open Patent Publication No. 2003-46186.
However, when input light power input to an SOA becomes small, the ratio of ASE power to signal light power in the output light of the SOA increases and a difference between output light power and signal light power is generated. Hence, even if the output light power is controlled to be at a given level, the signal light power is not controlled to be at a given level.
In addition, when the wavelength of signal light input to the SOA changes, the proportion of ASE power in the output light power of the SOA changes accordingly, and hence, the control for keeping the output light power at a given level has wavelength dependency.
FIG. 13 illustrates the difference between the output light power and signal light power versus the input light power of an SOA. The horizontal axis of FIG. 13 represents the input power of signal light input to the SOA, and the vertical axis represents the difference between the output light power and the signal light power of the signal light included in the output light of the SOA. In FIG. 13, the difference between the output light power and signal light power versus the input light power of the SOA is illustrated for three different wavelengths.
Referring to FIG. 13, the smaller the input light power, the larger the difference between the output light power and signal light power of the SOA. This is because, the smaller the input light power, the larger the ratio of ASE power to signal light power of the SOA output light. Hence, even when the ALC of the SOA performs control such that the output light power of the SOA is at a given level, the signal light power is not controlled to be at a given level.
Referring again to FIG. 13, the difference between the output light power and the signal light power is larger for a long wavelength than for a short wavelength. This is because the amplification is lower for longer wavelengths, and if the amplification is increased, the ratio of ASE power to signal light power of the SOA output light increases. Consequently, wavelength dependency is generated in the control of keeping the output light level at a given level, in the ALC of the SOA.